The economic constraints of both passenger and freight railroad traffic are moving the railroad industry to higher-speed vehicles and higher axle loads. The heavy axle loads and high speeds of modern freight trains produce high track stresses leading to quicker deterioration of track conditions. As a result, the demand for better track maintenance has also increased. Fast and reliable methods are thus needed to identify and prioritize tracks in need of maintenance in order to minimize delays, avoid derailments, and reduce maintenance costs.
The condition and performance of railroad tracks depends on a number of different parameters. Some of the factors that can influence track quality are track modulus, internal rail defects, profile, cross-level, gage, and gage restraint. Monitoring one or more of these parameters can improve safe train operation by identifying track locations that produce poor vehicle performance or derailment potential. Track monitoring also provides information for optimizing track maintenance activities by focusing activities where maintenance is critical and by selecting more effective maintenance and repair methods.
Track modulus is an important factor that affects track performance and maintenance requirements. Track modulus is defined generally as the coefficient of proportionality between the rail deflection and the vertical contact pressure between the rail base and track foundation. In some cases, track modulus can be expressed as the supporting force per unit length of rail per unit rail deflection. Track modulus is a single parameter that represents the effects of all of the track components under the rail. These components include the subgrade, ballast, subballast, ties, and tie fasteners. Both the vertical deflection characteristics of the rail as well as the track components supporting the rail can affect track modulus. For example, factors such as the subgrade resilient modulus, subgrade thickness, ballast layer thickness, and fastener stiffness can affect track modulus.
Variations in track shape and structural integrity present hurdles in the rail industry: both low track modulus and large variations in track modulus are undesirable. Low track modulus can cause differential settlement that subsequently increases maintenance needs. Large variations in track modulus, such as those often found near bridges and crossings, can also increase dynamic loading. Increased dynamic loading reduces the life of the track components, resulting in shorter maintenance cycles. A reduction in variations in track modulus at grade (i.e. road) crossings can lead to better track performance and less track maintenance. It has also been suggested that track with a high and consistent modulus will allow for higher train speeds and therefore increase both performance and revenue. Ride quality, as indicated by vertical acceleration, is also strongly dependent on track modulus.
In addition to track modulus, variations in rail geometry resulting from track defects can also affect track performance. The relationship between modulus and geometry is complex. In some cases, areas of geometry variations often correlate with areas of modulus variations and vice versa.
Finally, track deflection is also important. Track deflection is related to the applied loads, and the track modulus (and stiffness) is also an important factor. Deflection is defined as the ratio of applied load to track stiffness. More general, it can be defined as the vertical displacement of a single point of rail from the unloaded to the fully loaded condition.